The invention relates to a process for producing color images electrophotographically by means of toner image transfer. Color images are here to be understood generally as multi-colored representations, and in particular the reproduction, true to the original, of colored copy. Above all, these also include color images for color proofing methods.
Color proofing methods or color proofing systems are an aid in color printing. As a rule, color printing is here to be understood as four-color printing. They are used at various points in the production process of a color print: for checking the reproduction-photographic steps in the preparation of the color separation films, as samples or approval basis for the color printing client and as an orientation aid for the printer on the printing press.
Color proofing systems are mainly used in gravure printing and offset printing, and to a small extent in flexographic printing. They use the screened color separations of the colored original, such as are required for producing printing plates in individual colors.
Mainly photochemically operating color proofing methods are known. Currently the most widely used system uses a laminate, in which tacky monomers are photopolymerized imagewise and thus lose their adhesive holding power. Dry color pigments adhere to the areas which have remained tacky. For each color, a laminate film is applied, exposed and tonered. In other laminate processes, UV-light sensitive colored layers are transferred to a carrier material, exposed on the latter and decoated in the non-image areas or, depending on the process, in the image areas. There are also photochemically operating overlay systems, in which UV-light sensitive color films become transparent after contact exposure with the color separations and after decoating of the image-free areas. When superposed in the precise register, they give a visual impression of the colored original.
Electrophotography is also used for producing color images. Most important are here in most cases color copiers which, however, do not have to meet the stringent demands of color proofing method with respect to correct color reproduction and the degree of image resolution. Dry toner developers are preferably used here. For some time, however, electrophotographically operating color. proofing systems have also been offered and used. These use liquid developers. The electrophotographic color proofing methods hitherto disclosed conventionally operate with superposition of the toner images of the individual color separations. This is effected either via a temporary support or directly on the photoconductor layer. It is also known to copy all the color separations one on top of the other onto a photoconductor layer and to transfer them from the latter onto the final receiving material. Electrophotographically operating systems are also known in which exposure of the photoconductor is carried out by means of a laser digitally controlled with the phases of the image information.
Liquid developers are used in the hitherto disclosed, electrophotographically operating color proofing systems for the reason that their toner particles have a small particle size of less than 1 .mu.m as a rule. In this way, a very high resolving power is achieved. A disadvantage is, however, that an insulating dispersant, as a rule a branched liquid hydrocarbon, must be employed, the evaporation of which involves environmental problems. Moreover, the possible high resolution of the liquid developer does not manifest itself whenever other process steps, such as imaging itself or the transfer of the toner, are of lower quality. Frequently, high resolution, which would require particles of less than 1 .mu.m, is also not necessary. This is the case particularly if the printing plates for color printing are produced not by contact copying using film originals, but directly from an opaque original by optical imaging via a lens. This applies, for example, in the production of newspapers, where a relatively large number of plates must be produced within a short time. 48's screens are used here as a maximum (48 image elements per cm). As a rule, however, the screen is markedly coarser. Because of the great increase in colored images in newspapers, the question of an adequate color proofing system is here particularly acute. Even in poster printing, where colored reproduction predominates, a trend towards film-less plate production can be observed. In this case, the screen is even substantially coarser, but the formats are very large.
A color proofing system suitable for these user groups must be quick, reliable and inexpensive, but it must also allow processing of very large formats. By contrast, the resolution does not necessarily have to meet the most stringent requirements. The property patterns of the conventional electrophotographic toner image transfer processes do not meet these requirements.
When toner is transferred from the photoconductor layer to a receiving material or to a temporary support and from the latter to the receiving material, this must be done in precise register in the case of color images, so that the correct color impression is created. Moreover, defined quantities of toner must be transferred, since otherwise the depth of the corresponding colors is incorrect. In a conventional electrostatic transfer, especially when dry developers are used, the toner already transferred affects the transfer of the subsequent toner.
As an effective and evenly operating transfer principle, it is known to bring a two-dimensional material having a tacky surface into contact with the toner image located on the photoconductor layer. Since the non-image areas are then still tacky, the copy thus obtained can then be used only for restricted purposes. A variant is known in which the toner image is heated on the temporary support and the toner is then transferred in the molten state onto a final receiving material. The temporary support must here meet the requirement that it has the least possible interaction with the toner, so that the process becomes repeatable without disturbance. The toner must thus not be pressed into the temporary support.
It is also known to press toner images from dry developers into the receiving material by means of pressure. This is done mainly with one-component magnetic materials and with zinc oxide papers. However, because of the relatively low copy quality, this process has lost importance.